Membranes are the outer container of a cell which is comprised mainly of proteins and lipids. As such, the membrane of the cell has a vital role in the process of all living things. The membrane has processes for transporting molecules into and out of the cell. This transport process has important roles, and can be either active or passive. The active process requires energy to take place. The passive process includes diffusion, osmosis, and filtration. These transport processes regulate cell volume, maintain intracellular pH and ionic composition in a narrow range, extract and concentrate metabolic fuels and building blocks and exude toxic substances. They also generate ionic gradients for excitability of nerve and muscle.
As membranes play such a vital role in the processes of life, understanding and monitoring the different interactions with membranes provides information that can lead to improved health care. In the study of membranes or membrane fractions, it is important to remember that the internal and external component of the membrane are different.
Biological membranes have many components that are involved in regulating entry of molecules into or out of a cell, or in transmitting signals from messengers, such as hormones, into a cell. Such components can comprise proteins, such as G protein-coupled receptors, ion-channel receptors, tyrosine kinase-linked receptors and cytokine receptors. Identifying molecules that block or interfere with the interaction between these components and their natural binding partners is an important area of pharmaceutical research. The use of isolated membrane components for studies allows for concentration of the molecules, but also removes them from their natural environment. One challenge in using membrane bound components for such tests is the small amount of the component in a membrane: In order to be able to detect an interaction between a membrane component and a binding partner, both molecules must be in concentration above the sensitivity threshold of the instrument used for detection.
In certain methods, membrane material can be bound to a solid support. Such devices are described, for example, in U.S. Pat. No. 5,922,594 (Lot) and U.S. Pat. No. 7,045,171 (Bookbinder).
Back-scattering interferometry (“BSI”) is a method useful for detecting interactions between molecules in a sample. A version of the method was described in U.S. Pat. No. 5,325,170 (Bornhop et al., Jun. 28, 1994). The method described there involves directing a laser beam onto a channel to produce back-scattered light in the form of an interference fringe pattern. The form and location of the fringe pattern is a function of the refractive index of the liquid being interrogated. Binding events between molecules in the fluid, such as target-receptor interactions, change the refractive index of the fluid and result in a shift in the location of the fringe pattern. Detecting shifts in the fringe pattern is a way of detecting binding events in the fluid.
BSI allows for both homogenous assays, in which both binding partners are in free solution, and heterogenous assays in which one of the binding partners is tethered to the surface of the channel. U.S. Pat. No. 6,381,025 (Bornhop et al., Apr. 30, 2002) describes a method for performing back-scattering interferometry in which a channel having a generally hemispherical cross-sectional shape is disposed in a micro-fabricated substrate. U.S. Pat. No. 6,809,828 (Bornhop et al., Oct. 26, 2004) describes a chip for back-scattering interferometry in which the substrate has a channel taking the form of a rectangle. U.S. Pat. No. 7,130,060 (Bornhop et al., Oct. 31, 2005) describes a method for determining absolute refractive index using back-scattering interferometry in which light is directed at a capillary tube and refractive index is determined as a function of the angle at which there is a marked change in intensity. Bornhop et al., Science, 317:1732, Sep. 21, 2007, describes free-solution, label-free molecular interactions investigated by back-scattering interferometry.